One or more biologically active peptides made by thymic tissues induces T cell markers and functions in some lymphocytes. The thymus, presumably through these products, plays a major role in the normal maturation of the T lymphocyte series and, at least in experimental animals, also contributes to sustaining the T lymphocyte series. This laboratory has recently developed a bioassay that measures Thy 1.2 antigen induction on precursor cells from spleens of nude mice that are genetically programmed to express this thymus-dependent antigen. This simple and reproducible bioassay is applicable to testing thymic activity (as it applies to T lymphocyte induction) in human plasma. In this project, we wish to apply this new technique to the study of patients with thymomas and other thymic disorders. Baylor University is a referral center for patients with myasthenia gravis; at present over l00 patients that attend this clinic are available for study. We wish to correlate thymic activity in the plasma of individual patients with disease severity, thymic histology (if thymectomy is performed) and clinical benefit from thymectomy. Thymoma is a likely diagnosis in patients with anterior mediastinal masses. We wish to see if plasma thymic bioassay results prove diagnostically useful and in particular if neoplastic and benign thymomas can be distinguished from one another. Sternberg described an acute childhood leukemia that appears to directly involve the thymus--analogous to leukemia in AKR mice. We propose to study such children.